According to a conventional brushless motor controller and a control method, it is known to switch between energization modes for applying a pulse voltage to two phases in a three-phase brushless motor in accordance with a pulse width modulation (PWM) signal when the pulse induced voltage induced in a non-energized phase during the pulse voltage application crosses a threshold defined for each of the energization modes (see Patent Document 1, for example).
However, the pulse induced voltage oscillates immediately after the pulse voltage is started to be applied to two phases. Thus, if the duty of the PWM signal is reduced so as to rotate the brushless motor at a low speed, the pulse induced voltage might be sampled within the oscillation period of the pulse induced voltage, and thus the energization modes might be switched at wrong timing.
To address this, in the brushless motor controller and control method according to Patent Document 1, the pulse induced voltage in a non-energized phase is detected in a predetermined period (hereinafter referred to as “main period”) among multiple contiguous periods in the PWM signal while a duty (hereinafter referred to as “main period duty”) during the main period of the PWM signal is limited to a minimum duty so as to avoid the sampling of the pulse induced voltage within the oscillation period. In addition, a duty (hereinafter referred to as “adjustment period duty”) during each of periods (hereinafter referred to as “adjustment periods”) other than the main period among multiple contiguous periods in the PWM signal is set to a value smaller than the main period duty. Thereby, an average duty, which is an average value of the main period duty and the adjustment period duties, is adjusted to be equal to a duty (hereinafter referred to as “set duty”) set for control such as rotation speed feedback control.